Method of producing calcium carbonate single crystals

ABSTRACT

A method of producing calcite single crystals by floating-zone melting of a polycrystalline rod of calcium carbonate and an eutectic mixture, for example, of calcium carbonate and lithium carbonate which is introduced in the form of a pastille in contact with a seed crystal and which is displaced in the molten state in the direction of length of the rod subject to the action of a moving temperature gradient.

I United States Patent 1 3,663,180 Brissot 51 May 16, 1972 [54] METHODOF PRODUCING CALCIUM 2.990.261 6/1961 Greiner ..23/3o| 3,238.024 3/1966Cremer et al. ....23/301 CARBONATE SINGLE CRYSTALS 3,345.14! 10/1967Nester et a1 ....23/304 [72] inventor: Jean Jacques Lucien EmileBrissot, ,396, 9 8/1 8 amma m "23/301 M i Alfort France 3,484,30212/1969 Maeda et al. ...23/301 3,505,032 4/1970 Bennett ..23/301 [73]Asslgneez U.S. Philips Corporation, New York, NY.

22 Filed: Oct 22 Primary Examiner-Norman Yudkofi' Assistant Examiner-R.T. Foster [21] Appl. No.: 82,870 Attorney-Frank R Trifari 57 ABSTRACT[30] Foreign Application Priority Data I l A method of producing calcitesingle crystals by floating-zone Nov. 21, 1969 France ..6940146 memng ofa polycrysmmne [0d Of cakium carbonate and an eutectic mixture, forexample, of calcium carbonate and lithi- 2 6 um carbonate which isintroduced in the form of a pastille in [51] Int. Cl. ..B0lj 17/10, BOlj17/14 contact with a seed crystal and which is displaced in the mol-[58] Field of Search ..23/30l SP, 304, 66, 300 ten state in thedirection of length of the rod subject to the action of a movingtemperature gradient. [56] References Cited 5 Claims, 1 Drawing FigureUNITED STATES PATENTS 2,747,971 5/1956Hein.............e..........................23/301 0 Z EUTECTIC oPASTILLE PATENTEUMAY I 6 I972 l-i, F353 1 8 U OOOOOO 'CALCITE SEEDINVENTOR.

BY JEAN J.L.E. BRISSOT AGENT METHOD OF PRODUCING CALCIUM CARBONATESINGLE CRYSTALS The invention relates to a method of producingartificial calcite single crystals.

Doubly retracting, rhomboedric calcite single crystals are frequentlyemployed on an industrial or semi-industrial scale, inter alia indevices employing polarized light or as supporting crystal forpara-magnetic ions in lasers.

Since the natural locations of calcite, particularly on Iceland, arebeing exhausted and large crystals are of undiminishing interest intechnology, it has been endeavoured to produce such crystalsartificially.

From British Patent specification l,049,347 a method of cultivatingcalcite crystals is known, in which calcium carbonate together with amelting agent, for example, lithium carbonate, or a mixture of lithiumcarbonate and potassium carbonate, the quantity of calcium carbonateexceeding the quantity corresponding to the eutectic composition, isheated in a carbon dioxide atmosphere so that at least a portion thereofmelts without decomposing, after which at least a portion of the melt iscooled so that calcium carbonate crystallizes in the form of calcite.

By heating the eutectic mixture with calcium carbonate in the presenceof at least one seed, calcite is, indeed, obtained in single crystalform after cooling, but these crystals are surrounded by the solidifiedeutectic composition. The chemical properties thereof and those of thecalcite are similar to each other to an extent such that it is, inpractice, not possible to isolate the crystals from the surroundingmass.

The present invention provides an elegant method of ob taining large,substantially pure calcite single crystals.

According to the invention the method of producing calcite singlecrystals with the aid of a seed crystal by melting and coolingpolycrystalline calcium carbonate in the presence of an eutectic mixtureis characterized in that the single crystal is obtained by floating-zonemelting of a polycrystalline rod, while the eutectic mixture introducedin the form of a pastille in contact with the seed crystal is displacedin the molten state in the direction of length of the rod under theaction of a moving temperature gradient.

Heating of the polycrystalline rod and formation of the single crystalare thus performed simultaneously. The rate of displacement of the zonecorresponds with the rate of formation of the single crystal. The latterrate may lie between 50 and 500 m an hour at a temperature of the moltenzone between 700 C. and 900 C. Under these conditions the melting agentin the molten zone is constantly urged by diffusion towards theinterface.

The rod of calcium carbonate to be used may be made by compressing finepowder with a very small quantity of solvent.

Suitable eutectic mixtures are inter alia the system CaCO Li,CO mixturescontaining at least one alkali salt such as K,C KCl LiCl and lithiumcarbonate Li,CO,. Other metal salts may be added to the sintered calciumcarbonate in order to produce doped crystals or mixed carbonates such asmixture crystals of calciumand barium carbonate.

One example will now be described with reference to a drawing whichshows schematically a device for growing artificial calcite crystals.

Seed crystal 1 is arranged in a holder 2, which is set rotating veryslowly by means of a motor (not shown). On top of the seed crystal 1 apastille 3 of the eutectic composition is arranged. On top of the pastil3 is arranged the lower end of a rod 4 formed by a sintered calciumcarbonate cylinder, which is held in vertical position coaxially withthe seed crystal 1 by means of a holder 5. This holder 5 is slightlydisplaceable along its axis so that the molten zone can be contracted orenlarged in order to compensate for the capillary forces during thegrowing process. (This mechanism is not illustrated).

The holders 2 and 5 are connected with each other by means of a bracket6, which is vertically displaceable along screwthread 7; thescrew-thread is adapted to be turned with the aid ofa motor (not shown).I

The pastille 3 can be melted by irradiation with the aid of aheating-spirals, or preferably, by a metal tape 8, for example, ofgold-plated platinum having small apertures at the center through whichthe melt can pass. The latter embodiment is shown in the drawing.

The tape 8 is heated by direct passage of current via the stationaryelectrodes 9.

In order to avoid thermal stress the auxiliary spirals l0 and II areprovided to hold the crystalline rods at an appropriate temperature. Theentire device is enclosed by an envelope 12 in which an excess pressureof carbon dioxide gas is constantly maintained in order to avoiddecomposition of the calcium carbonate during the heating process. Theenvelope i2 is shown only schematically on the drawing and, as a matterof course, its dimensions are such that the bracket 6 is displaceableover a length at least equal to that of the rod 4.

The growing process includes melting of the eutectic pastil 3 by meansof the heating tape 8 and the spirals l0 and 11. When the melt hasuniformly wetted the seed crystal 1 and the rod 4, the bracket 6 ismoved very slowly downwards, whereas the tape 8 remains at its place Themolten zone is thus displaced relatively to the seed crystal in thedirection towards the holder 5 and it leaves a calcite single crystalbehind.

The movement of the bracket 6 may be reversed, the seed crystal thenbeing arranged above and the rod 4 below in the device.

This method permits of displacing a small quantity of melting agent andof obtaining a crystal not embedded in the melting agent.

WHAT lS CLAIMED IS:

1. A method of producing calcite single crystals comprising the steps,placing a seed crystal of calcium carbonate and a pastille of aneutectic mixture in contact with a polycrystalline rod of calciumcarbonate, heating the polycrystalline calcium carbonate rod to melt thesame, displacing the pastille in contact with the seed crystal in thedirection of length of the rod under the action of a moving temperaturegradient, and thereafter cooling the rod to form a single crystal ofcalcium carbonate.

2. A method of producing calcite single crystals as claimed in claim 1wherein the rod is passed through a heating elec' trode having aplurality of apertures.

3. A method of producing a calcite single crystal as claimed in claim 1wherein the rod contains another metal salt.

4. A method of producing a calcite single crystal as claimed in claim 3wherein the rod contains as another metal salt a carbonate of anotheralkaline earth metal.

5. A method of producing a calcite single crystal as claimed in claim Iin which the eutectic mixture is CaCO, LiCO,.

2. A method of producing calcite single crystals as claimed in claim 1wherein the rod is passed through a heating electrode having a pluralityof apertures.
 3. A method of producing a calcite single crystal asclaimed in claim 1 wherein the rod contains another metal salt.
 4. Amethod of producing a calcite single crystal as claimed in claim 3wherein the rod contains as another metal salt a carbonate of anotheralkaline earth metal.
 5. A method of producing a calcite single crystalas claimed in claim 1 in which the eutectic mixture is CaCO3 - LiCO3.